[Kst] Extended selection mode for editmultiplewidget

[Brisset, Nicolas]

> Qt offers both Multi and Extended mode for various widgets.  
> The default is single selection mode.
In fact, there are four modes: no selection (obvious, right ?), single
(only one item can be selected), Multi (many items can be selected, but
they need to be selected one at a time: that's the current behavior) or
Extended (which I find better for selections with potentially large
numbers of items). 
I think the default is Single indeed, but that's clearly not appropriate
in a "Edit multiple" widget :-)

Nicolas